"Cancelable" streams

[Brian Goetz]

> How often is the BooleanSupplier called?

This is a tradeoff of responsiveness for overhead.  It could be as often 
as every element; it could be as infrequently as starting every new split.

> Can one implement a BooleanSupplier that depended on the number of
> elements generated?  (Or would you just use a lazy range method instead?)

With some user responsibility for thread-safety, yes.  For example:

   Collection answers = new ThreadSafeCollection();
   stream.cancelOn(() -> answers.size() >= 10), () -> {})
         .filter(...)
         .forEach(answers::add);

It would be the user's responsibility to ensure that access to the 
shared data is properly protected.

> Doesn't cancellation occur as soon as the supplier returns false?  If
> so, what's the advantage of an onCancel method?

The onCancel lambda is so that there can be a feedback mechanism by 
which the client can answer "did my pipeline complete because it ran out 
of elements, or because we stopped processing due to cancelation?"